Australia is an island continent with a distinctive and unique flora
unlike that found on any other land mass. The evolution of its plants was a
result of Australia's long isolation from the other continents.

The
story starts in the cretaceous period (136 Ma) when the flora of the world
began localising, particularly in the southern continents (Australia,
Antarctica, India, South America and Africa) due to the splitting of
Gondwana. (See the section on continental drift).

In Australia, there
was a great flood during the Cretaceous period which broke the continent
into four distinct islands. Between these islands were shallow seas, which
supported extensive marine life. The existence of that marine life created a
lot of calcium carbonate (CaCO3) and when this broke down, mixed
with the soils (seabeds at the time) to give them an alkaline property. The
'islands' from this era (the part of Australia that was never flooded)
maintained their acid soils.

Direct evidence for this island
theory comes from the fact that many molluscs and other fossilised sea
creatures have been found in outback Australia, as well as the alkaline nature
of the soils.

During the massive floods, the flora on each island
diversified and evolved with each island developing its own unique flora. This
was partially due to the long periods of stability that arose.

During the floods of Australia, were three islands of key importance:

South Western Australia

Adelaide Region (South Australia)

South-Eastern Australia (Victoria & New South Wales)

The deposits of limestone that were created between these islands meant that
once the flood was over there was a sharp contrast between the pH of the soils, and
the flora of the islands could not readily move from one soil type to the other.

This forced the process of evolution to speed up so that new plants could
colonise the new land areas.

Between the flooding and the most recent
drought in Australia, there have been four Ice Ages, the first being 500 000
years ago, and the last being 10 000 years ago. In some parts of the
Northern Hemisphere, there was ice up to 3000 metres (3 kilometres) thick,
however Australia only had ice to a maximum depth of 30 metres in Tasmania
and Mount Kosciusko (Australia's largest mountain).

In Europe many
species were wiped out, as they had nowhere to 'travel'; in Asia and North
America, the distribution of plants gradually moved south and then
re-entered the north once the climate became warmer, reducing the number of
extinctions. (Of course, there were plants which for varied reasons could
not shift their distributions, and became extinct). Because Australia was
largely unaffected by these Ice Ages, there was minimal impact on the flora.

However, roughly 5000 years ago there was a long-lasting major drought
across the Australian continent which lasted for several centuries. During
this time there were extinctions, as well as the creation of dunes. Soils
became mobile.

There were a few wetter pockets that were unaffected
by these climatic changes, these being:

The Grampians

Mount Buffalo

Flinder's Ranges

This lead to a greater diversity of flora across the continent as plants
adapted to the changed climates.

It is thought that it was in these
dry periods that the genus Eucalyptus (Gum Trees) which are perhaps
one of the most widely recognised Australian plants, changed from being a
comparatively minor component of the landscape to being a major component.

Though the aforementioned drought would have helped in the spread and
diversification of Eucalyptus, its major dominance started
considerably earlier than a mere 5000 years ago!

Of course, there are
many more genera unique to Australia than Eucalyptus:

Australian flora is relatively unique.

80% of Australian native species are found only in Australia

30% of Australian native genera are found only in Australia.

Being a southern land mass, Australia was thought of by many in bygone times
as being a nasty desolate country.

Because palaeobotanists are only
human - affected by the same prejudices as everyone else, they decided that
the Australian flora could be classified into three groups:

Indo-Malaysian - the northern rainforests

Antarctic - southern rainforests including
Nothofagus spp.

Australian - Eucalyptus & Acacias

Part of the reason for this classification was that "nothing nice could
possibly have come from Australia". Hence all the "nice" vegetation had to
have come either from the north (Malaysia, Indonesia, Asia) or from the
south (Antarctica). This theory makes no sense.

Australian plants are
now classified as either being:

Relictual - rainforest remnants of Gondwana
(now closed forests)

Australian - localised flora

It was from the relictual forests that the Australian flora evolved.

There are similarities between the Asian rainforest (that is India - not
China, Japan etc) and Australian rainforest that suggest a common ancestry.
A survey conducted by Dr. L. Webb found that Australian and Indian
rainforests shared 47 genera, but only 41 with Papua New Guinea (which was
attached to Australia until relatively late).

So there is a foreign
influence - a shared ancestry. And of course there are some migratory
species such as some orchids which were introduced from wind-borne seed.

In the Tertiary period (65 Ma) the climate was moist and mild. The
entire Australian continent was probably covered by a subtropical
rainforest-type vegetation. This is sometimes referred to the pan-Australian
flora.

As mentioned earlier, this forest was evolved from the
Gondwanan rainforests. These forests were also the predecessors of the
forests of India, Madagascar, Africa and South America.

Evidence for
the theory that most of Australia was covered in rainforests comes from
fossils of Australia's only native holly - Ilex arnhemensis. I. arnhemensis
today only grows in the lowland forests of northern Australia, however
fossils of this species have been found all over Australia.

As the
climate changed, so to did the vegetation. While in some areas the
sub-tropical vegetation (Gondwanan rainforests) persisted, in other areas a
cool temperate rainforest evolved. And of course in many areas where the
soil was nutrient-poor, the open forests containing Eucalyptus and
Acacia
species developed.

Suggestions have been made that Australia was the
origin of the Gondwanan rainforests, owing to the high number of primitive
flowering and other plants which still live in these forests (particularly
in Queensland). In fact, Australia has the highest concentration of
primitive plant families in the world. Of the 19 plant families that are
thought to be the most primitive, 13 are found in Queensland.

Psilotum nudum is a native of Queensland, Australia and is an ancient
Rhynia-type (Psilophyta) plant which is still growing there today. It
is one of the oldest vascular plants living on earth.

It is now
accepted that the sclerophyllous (dry-country) vegetation that dominates
most of the Australian continent evolved from these Gondwanan forests.

It was about 16 Ma in the late Tertiary period that the arid-zone
Australian flora evolved, including the sclerophyllous flora.

Most of
the Australian sclerophyllous flora evolved from the Gondwanan forests
including Eucalyptus and Acacia, however the Proteaceae evolved from the
Antarctic portion. (Proteaceae includes the common Australian genera
Banksia, Grevillea, Hakea, Isopogon,
Buckinghamia and Dryandra amongst others. Proteaceae evolved
on Gondwana and grew in what are now known as Africa, South America,
Australia, Antarctica and Madagascar.

Once the Eucalypts and acacias
began to dominate, the rainforests declined. This was partially due to the
increasing incidence of fire in the Australian landscape. Since the acacias
and eucalypts adapted well to these conditions, they thrived. And since
there was differences in the soils and environments across the Australian
continent, they diversified readily. There are now 800 recognised species of
Eucalyptus and 900 species of Acacia in Australia.

Of
course, there was human influence also. There is a widely-accepted theory
which states the Australian aborigines contributed to the change in the
Australian landscape.

Aborigines are thought to have colonised
Australia about 38 000 years ago. Incidentally, the charcoal deposits in the
fossil record increase at around this time. It is known that the aboriginals
had a régime of burning, which led to the renewing of the Australian bush.

While fire had already restricted the rainforests of Australia, it
is thought that the aboriginal fire program in combination with the dry
climate may account for the unusually high levels of sclerophyllous
vegetation in Australia. It is also though that the high frequency of fires
eliminated the Araucarian forests of Australia, and restricted Dacrydium to
Tasmania.

In summary, it is thought that the aboriginal program of
burning gave rise to the following changes in the Australian landscape:

The elimination of forests in many areas

The prevalence of fire-resistant and fire-dependant species, and
species which became tolerant of the fire regime

The dominance of sclerophyllous vegetation

The common occurrence of grassy understoreys.

The arrival and
settlement of Europeans in 1788 also wrought significant changes,
particularly agricultural practices which led to:

Soil
erosion

Salinity

Soil compaction

Waterway pollution -
blue-green algae

The destruction (and extinction) of significant
numbers of native plants, either through overgrazing (from introduced
animals) or from the introduction of new plant species which out-competed
native plants. New plant species were introduced, some becoming severe weeds
such as the Prickly Pear. There is no other place where the changes have
been so fast, extensive, and destructive.

Knowing and understanding
the evolution of the Australian flora can help everyone appreciate the need
to ensure its protection. Protecting small pockets of forest is no good as
these are prone to attack from foreign species. Large passages of land are
what's needed.